The Collaborative International Dictionary of English v.0.48:

Orchestration \Or`ches*tra"tion\, n. (Mus.)
   The arrangement of music for an orchestra; orchestral
   treatment of a composition; -- called also instrumentation.
   [1913 Webster]

WordNet (r) 3.0 (2006):

orchestration
    n 1: an arrangement of a piece of music for performance by an
         orchestra or band
    2: the act of arranging a piece of music for an orchestra and
       assigning parts to the different musical instruments [syn:
       orchestration, instrumentation]
    3: an arrangement of events that attempts to achieve a maximum
       effect; "the skillful orchestration of his political
       campaign"